The present invention relates to a duplex stainless steel which has improved strength and corrosion resistance in chloride-containing environments and which is particularly suitable for use in applications where conventional duplex stainless steels may undergo corrosion such as in heat exchanger tubes, line pipes, and similar products, and in applications where high strength is required for reduction of material cost or weight.
Duplex (ferritic-austenitic) stainless steels have good corrosion resistance, particularly in sea water and they have been used for many years in various industrial equipment including heat exchanger tubes. Many attempts have also been made to improve duplex stainless steels, as proposed in Japanese Patent Applications Laid-Open Nos. 50-91516(1975), 52-716(1977), 56-142855(1981), 62-50444 (1987), 62-180043(1987), and 2-258956(1990).
In recent years, as the environments in which corrosion-resistant metallic materials are used become more severe, these materials are required to have higher levels of corrosion resistance and superior mechanical properties. Duplex stainless steels are no exception. In order to meet such requirements, the so-called super duplex stainless steels have recently been developed. For example, see U.S. Pat. No. 4,765,953; Vernhardsson, S., Corrosion 90, Apr. 23-27, 1990, Paper No. 164; and Lefebvre, G. et al, Proceedings of the First (1991) International Offshore and Polar Engineering Conference, pp. 224-232.
Pitting resistance equivalent (abbreviated as PRE or P.I.) of a duplex stainless steel which is defined by the following formula (b) is known as a parameter indicating resistance to localized corrosion, particularly to pitting corrosion: ##EQU1## where the percent of each element is by weight.
In general, the Cr, Mo, and N contents of a duplex stainless steel are adjusted in such a manner that the steel has a PRE of 35 or higher. The super duplex stainless steels have a PRE above 40 by further increasing their Cr, Mo, and N contents and they are attracting interest as materials having excellent corrosion resistance, especially in sea water. The increased Cr, Mo, and N contents of super duplex stainless steels lead to an increase in strength. Therefore, the strength of super duplex stainless steels is even higher than conventional duplex stainless steels which inherently have a higher strength than ferritic or austenitic single-phase stainless steels, which is another prominent feature of super duplex stainless steels.
As described above, the basic concept of alloy designs for super duplex stainless steels, which surpass conventional duplex stainless steels in respect to corrosion resistance and strength, resides in increased contents of Cr, Mo, and N. However, when added in increased amounts, these elements give rise to the following problems.
The addition of Cr and Mo to a duplex stainless steel in increased amounts tends to cause the formation of hard and brittle intermetallic compounds called .sigma.-phase, .chi.-phase, Laves phase, and the like (hereinafter referred to as .sigma.- and similar phases). As a result, the steel becomes difficult to work and flaws and cracks may be formed during working, thereby making it difficult to industrially manufacture steel products such as tubes in a stable manner. An excessive increase in the N content causes a deterioration in mechanical properties due to the formation of nitrides and generation of blowholes. Furthermore, when a duplex stainless steel having increased Cr and Mo contents is welded, intermetallic compounds (.sigma.- and similar phases) are precipitated in the steel by the effect of heat generated during welding, resulting in a deterioration in not only corrosion resistance but also in mechanical properties such as toughness and ductility in heat affected zones. Since the thermal structural stability of the steel is degraded in this manner, strict control of heat input during welding and heat treatment after welding are necessary in order to avoid such degradation, leading to a decrease in operating efficiency when steel tubes or other products made of the steel are installed.